I was wondering if the high heat indexes experienced during the summer months impact operations at DFW and IAH. Are aircraft, particularly those
flying long-haul routes, subject to restrictions due to the high heat? Does
any airline take precautions to protect aircraft and fuel systems from the
heat?

The airplanes are in the air most of the time. Every minute a plane is on the ground is revenue lost for the airline. It's not like the planes are sitting around the tarmac for hours in 107 degree heat, as our cars are.

Experts please correct me if I'm wrong, but I don't think heat is as much a problem as humidity. The more moisture in the air, the heavier it is meaning aircraft require longer takeoff distances in order to achieve lift, as opposed to when the air is dry.

DFW (and JED for that matter) both have very long runways which can allow a fully-loaded 747 to take off under such conditions.

Tuesday was burning! I don't think it really effects the airplanes that much(unless if you are AA and CO and WN) where you have maintenance bases in texas.As busy as DFW is and the fact that sometimes getting to the runway(distance) takes longer than the actual flight seriously,I don't think it effects the aircraft that much unless if the airplane is sitting on the ground for 4 hours +.it might effect them then but if you are talking about the widebodies(KLM,Sabena,British Airways,KAL,JAL LH,AA,CO etc) i don't think it effects them that much,i think the only thing you can really do is try and get a quicker turn around(load baggage,load meals,clean airplane etc) and get the passengers on board and seated ASAP,It may not be a problem especially since IAH and DFW are pretty big airports with several runways. My question is how the ground crews handle it!
blink182

Actually, humidity decreases performance by decreasing air density. Water vapor weighs less per unit volume than air. The water molecules displace the air molecules, making the air less dense. As air density drops, so does engine and airfoil performance. High temps also decrease air density. I don't think this is much of a problem for larger aircraft, as their engines are usually derated and have a fair amount of power to spare. Regional airlines at DFW and IAD may be feeling the pinch, though. Garrett turboprops like those on the Jetstream 31, 32 and 41 are notorious for their poor hot weather performance. I understand that the RR/Allison engines on the Embraer RJs also wimp out when the mercury climbs. I had to shake my head the other day when I heard that the temp in the Dallas area was around 110 (not unheard of) but the dewpoint was 85! To my brother mechanics in the Southeast- MOVE! Come out West where at least it's dry.